1. Field of the Invention
The present invention relates to locking systems for circuit board holding frames and more particularly to a self-locking system for securing circuit boards in a circuit board holding frame thereby preventing the boards from becoming inadvertently disconnected from the frame's electrical system.
2. Description of the Prior Art
It is conventional for circuit boards to be held in a circuit board holding frame so that the circuit board connectors remain in mating contact with mating frame connectors mounted on the back portion of the frame. In order to obtain the necessary spacing from board to board, guide elements, in which the boards are slideably inserted, are provided and secured to the frame. The guide elements provide the necessary alignment to insure that the board contacts make a solid connection with the contacts in the mating frame connectors. It is essential for proper circuit operation that the board contacts and the mating frame connector contacts remain solidly connected.
Problems can occur when seismic motion or vibration caused by transit or local traffic causes the circuit boards to "walk out" of the circuit board holding frame. This occurs even though a substantial amount of force is usually necessary for disengagement of the board and mating connectors. As a result, the contacts of the circuit boards completely lose the connection with the mating frame connector contacts. In the alternative, the board contacts may retain a connection which is not solid but intermittent and, therefore, not dependable. This naturally causes problems for consistent system behavior.
This "walk out" problem has been solved, in some instances, by utilizing retaining mechanisms on each board. For example, screws may be used for securing each board to the circuit board holding frame. This method, however, is rather tedious and time-consuming, especially when the insertion or removal of circuit boards is somewhat frequent.
Other systems have been designed to solve the "walk out" problem while making it easier for the user to install and remove circuit boards. One such system utilizes a retaining bar extending from one side of the circuit board holding frame to the other across its opening. The retaining bar, which is pivotally mounted at each end to the frame, may be rotated from a "locked" position to a "loading" position by moving it upwardly and backwardly. In the locked position, the retaining bar blocks the frame opening and prevents the circuit boards from moving outwardly from their mating frame connectors. Thus, the retaining bar insures a solid connection at all times. A spring retainer is utilized on one side of the frame to hold the retainer bar in its locked position.
In the loading position, the retainer bar is positioned out of the way of the circuit boards so that they may be removed or installed in the frame. To move the bar from the locked to the loading position, one hand is used to push the spring retainer outward while the other hand is used to push the retainer bar up and backward into the loading position. This provides clearance for the removal or installation of the circuit boards. To lock the system, one hand is used to push the spring retainer sufficiently outward to allow the retainer bar to return to its locked position while the other hand is used to pull the retainer bar forward and downward. The spring retainer is then released in order to secure the retainer bar in the locked position.
With this system configuration, however, the spring retainer provides a positive locking force only on the portion of the retainer bar adjacent to the spring retainer. At the opposite end of the retainer bar, there is naturally less locking force exerted by the spring retainer and the retainer bar may move to a limited extent if a force is exerted on it. Thus, even by using this system, the circuit boards furthest from the spring retainer can "walk out" of their mating frame connectors and become disconnected from the system because the retaining force exerted is insufficient to counter a force exerted by a "walking" circuit board. This is especially true for wider circuit board holding frames and heavier circuit boards.
While it is possible to equip the above-described system with a spring retainer on each side of the frame in order to achieve sufficient retaining force on all of the circuit boards in the frame, the use of two spring retainers would require two hands to unlatch the device and would create a handling problem for the user. In addition, such a system requires many different parts thereby complicating the manufacture and assembly.
Furthermore, in both the single and double spring retainer systems, the retainer bar intrudes into space in front of and above the circuit board holding frame when it is in the loading position. This space is normally allocated to another circuit board holding frame or electronic device and may or may not be available for such use.
Finally, this type of system does not have a "fail-safe" feature. More specifically, in order to lock the circuit boards in place, the user must bring the retainer bar back to its locked position. This requires a positive action by the user encompassing the pushing of the spring retainer sufficiently outward so that the bar may be moved from the loading to the locked position. If the user forgets to lock the retainer bar, the retainer bar will remain in the loading position, thereby providing no positive locking on any of the circuit boards, thus losing its functionality.